Implantable cardioverter-defibrillators (ICDs) have evolved significantly since their clinical introduction by Miroski in 1980. Initial ICDs were designed to recognize ventricular fibrillation and to deliver high-energy shocks in an attempt to treat the arrhythmia. However, clinical electrophysiology research indicated that an ICD capable of recognizing and treating ventricular tachycardias as well as ventricular fibrillation was useful for prevention of arrhythmic death.
Subsequent ICD development lead to devices that were able to treat ventricular tachycardias with antitachycardia pacing and low-energy cardioversion shocks in conjunction with back-up defibrillation therapy. These ICDs monitored the heart rate and the onset of the ventricular arrhythmia from ventricular endocardial signals to determine when the heart was in need either of cardioversion to treat a ventricular tachycardia or of defibrillation to treat ventricular fibrillation. While it was successful in detecting ventricular arrhythmias, the ICDs were unable to reliably discriminate sinus tachycardia and atrial arrhythmias, particularly paroxysmal atrial fibrillation, from malignant ventricular rhythms because of the sole reliance on ventricular cardiac signals to determine the cardiac state. As a result, the ICD might deliver inappropriate therapy based on aberrant ventricular signals that have their origins in an undetected supraventricular tachyarrhythmia, leading to an uncomfortable cardioversion shock being delivered to the patient.
In an attempt to correct this problem ICDs have been designed with dual chamber sensing capabilities to detect and analyze both ventricular and atrial endocardial signals. This increase in cardiac signal input to the ICD has provided an opportunity to determine the origin and the nature of the ventricular tachyarrhythmia, and to reduce the frequency of inappropriate therapy being delivered to an implant patient. However, while the combination of antitachycardia pacing with low and high energy shock delivery as well as backup bradycardia pacing in ICDs has expanded the number of clinical situations in which the device may appropriately be employed, means of coordinating ventricular and atrial rate information in a way that results in a system that effectively and efficiently treats an implant patient is still desired.